Explosive initiator



Nov. 24, 1964 W. R. BROCKWAY ET AL EXPLOSIVE INITIATOR Filed June 11, 1962 all...

FIG. 3

FIG.4

ATTORN United States Patent 3,158,0h7 EXFLQSBZVE TNETEATGR William llrocltwa'y, Thorofare, and George A. Noddin, Mantna, NJL, as -gnors to F du Font de Nemours The present invention relates to explosive initiators and more particularly to explosive initiators so encased and of such physical characteristics that the effective forces of their explosive action are largely directed in a predetermined direction.

The art has long recognized the dangers inherent in conventionally used explosive initiators. Commercially available initiators contain from 6 to 13 grains of a highenergy detonating composition, loadings great enough to cause severe damage to the operator and surrounding equipment should accidental actuation of the initiator occur. However, since the success of any explosive opera tion depends to a large extent upon the reliable initiation of the explosive charge, there has been a general reluctance to decrease the explosive loading of the initiator charges. Prior attempts to devise safer initiators have involved highly complicated assembly techniques and complex ignition devices.

In accordance with this invention, we now provide an initiator or" improved safety and reliability characteristics which is capable of initiating even relatively insensitive explosives and which utilizes conventional bridge plug assemblies, safety fuse, and low energy connecting cord. Our novel initiator comprises a tubular shell having two extremities, one of the extremities being integrally closed by a closure member of lesser thickness than the sidewalls of the shell, and an ignition assembly inserted into and closing the other extremity, the shell having positioned sequentially therein from said integral closure member, a base charge, a priming charge, and an ignition charge, said base and priming charges each being at a loading of up to about 1 grain, and the combined columnar length of said base and priming charges within said shell being at least about 4 inch.

In a preferred embodiment of this invention, the loading of the detonating charge is 0.5-6.7 grain and the loading or" the priming charge is 0.3 to 0.7 grain, and the ratio of the depth of the combined layers of priming and detonating charges to the diameter of these layers is about 1.5 to 3.0.

To facilitate the use of conventional bridgeplug asser blies, safety fuse, and low energy connecting cord as the ignition assembly in the initiator of this invention, that portion of the shell surrounding the ignition assembly in a particularly preferred embodiment of our invention has a diameter greater than that of the portion surrounding the base charge and the priming charge.

Conveniently, in one preferred embodiment of this invention, the initiator is adapted to provide a delayed firing action by positioning a delay element of the conventional type between the ignition and the primin charge.

In order to describe the initiator oi this invention in greater detail, reference is made to the accompanying drawings wherein:

FIGURE 1 is a vertical, cross-sectional view of an initiator in accordance with this invention wherein the ignition assembly is electrically actuated.

FIGURE 2 is a vertical, cross-sectional view of a nonelectrically actuated initiator in accordance with this invention, and

FIGURE 3 is a vertical, cross-sectional view of an electrically actuated delay initiator in accordance With this invention. 5

FIGURE 4 is a modified form of the electrically actuated delay initiator of FIGURE 3.

The electric initiator of FIGURE 1 includes a shell 1 having a closed extremity 2 of lesser thickness than the side walls of the shell. The portion 3 of the shell adjacent the closed end is of lesser diameter than the remaining portion of the shell 1. Within the portion of this shell of reduced diameter is a base charge 4 and pressed into a conical cavity in this charge is a priming charge 5. The ignition assembly in this particular unit consists of a high resistance bridgewire d imbedded in a loose mass of ignition composition 7 and attached to and held in position by lead wires 8 which are in turn held in place by, and pass through, a sealing plug d of a resilient material. Peripheral crimps 13 hold the plug in place.

In FIGURE 2, the elements are as in FIGURE 1, with the exception that the ignition assembly in this particular unit is a low-energy connecting cord as described in US. Patent 2,932,219 having an explosive core it), a metal sheath l1, and countering 1.2. One end of this cord is insorted into the open extremity of the shell, so that the inserted end is adjacent the priming charge 5, and the shell 1. is crimpingly engaged about the portion of the cord in the shell.

In FIGURE 3, the elements are again as in FIGURE 1 with the exception that a delay element, consisting of a charge of a defiagrating composition 14 encased in a heavy-walled metal tube 15, is interposed between the ignition assembly and the priming charge 5.

The elements of FIGURE 4 are as in FIGURE 3; however, the lesser diameter portion of the shell contains the delay assembly.

The operation of the initiator of the invention is identical to that of conventional initiators, i.e., the priming charge is actuated by the burning ignition charge (which is ignited by a hot bridgewire or spark from safety fuse or an arc gap) or the delay charge when present, or by the detonation of the connecting cord, and initiates the base charge. The initiating impulse generated by the detonation of the base charge is concentrated axially downward due to the decreased resistance offered by the thin bottom of the shell and the depth of the charge with relationship to its diameter, so that the initiating effect at 2 of the assembly is at least as effective as that of initiators containing from 6 to 13 grains of detonating explosive as the base charge. The depth of the base charge in relationship to its diameter is critical-if the amount of base charge in a conventional initiator is reduced, a very thin layer of the explosive composition results, and energy from its detonation is spread over a larger area. Consequentially the initiating impulse is inadequate even if the bottom of the shell is thinner than the sidewalls. In our initiator wherein the ratio of the combined depth of the base and priming charges to the diameter of the charge is preferably from about 1.5 to about 3.0, the energy from the detonation is concentrated in a small area so that the initiating impulse is focused for maximum. effect without dissipation of the initiation impulse. In the present assembly, the side walls may vary in thickness from about (1.005 to about 0.630 inch, but the bottom thickness should not exceed 0.017 inch.

The following examples illustrate specific embodiments of this invention. These examples are to be understood to be illustrative only and not limiting the invention in any way.

Example I A series of initiators was assembled as shown in FIG- URE 2. The shells were of aluminum, with a 0.017 inch Wall thickness, and a closed extremity thickness as set forth in Table I. The inner diameter of a 0.313 inch long portion adjacent the closed extremity was 0.125 inch. The inner diameter of the remaining portion of the shell 3 was 0.20 inch. The total height of each of the shells was 0.9875 inch. Into these shells were loaded base charges of pentaerythritol tetranitrate (PETN) pressed at 50 lbs.,dead Weight, and prir'ning charges of lead azide. In each case, the depth of the'combined charge was 0.313 inch, the depth 'of the reduced diameter portion of the shell. A six inc'h length of low-energy connecting cord having an explosive loading of 078 grain of lead azide was 'crimped into each shell. The detonating power of these initiators was tested by embedding each of the initiators into-a 'cut stick of dynamite comprising 9.0% nitroglycerine, 69% ammonium nitrate, 3.0% sodium nitrate, 10% salt, 2% sulfur, 1.7% pulp, 4.8% meal and 0.5% chalk. The results of testing the initiators is given in Table I.

' Loading at 50 lbs. with Bottom Flat Pin (grains) Test Number Thickiiless, Result inc Lead Azidc PE'IN 0.016 1.0 D 1 0.016 0.6 0. D 0.016 0.2 0. 5 D 0. 005 0. 5 0. 5 D 0. 005 0. 2 0. 5 D 0. 003 1.0 0 D 0.003 0. 5 0. 5 D 0.003 0. 4 0. 5 D

D =Dynamite detonated Example 2 A series of electric initiators was prepared similar to those shown in FIGURE 1. The sidewalls of the shell were of 0,017 inch'thick aluminum whereas the aluminum closing one end of the shell was 0.003 inch thick. The outer diameter of the 'upper'portion of the shell was 0.260

inch. The outer diameter of the lower, closed extremity of the shell, 0.25 inch high, was 0.125 inch and its inner diameter was 0.108 inch. Into thissmaller diameter portion ofthe shell 0.6 grain of peritaerythritol tetranitrate was loadeda's'a base charge, and, above this charge was placed 0.5 grain of lead azide, each charge being pressed into position in the reduced diameter portion of the shell by 50 lb. pressure. The ignition assembly inserted into the larger diameter portion of this initiator consisted of 'a 0.0019 inch diameter nickel-chromium bridge "wire 'testi'ngequipment consisting essentially of a source of var- Y iable voltage and a seriesof picofarad condensers ranging in capacitancefrom 250 to 2000 picofarad, and the shell was connected to a ground line. Voltages from 0 to 30,000 volts were applied to a condenser of known capac- 'itance in increments of 2000 volts, and the condenser was allowed to discharge through the initiator. The initiator detonated when a voltage of 26,000 volts applied through a condenser of 250 picofarad was discharged 'throughit, indicating'that the initiator has a static resis- -'tanc'e great'er than 24,000 man equivalent volts (m.e.v.').

[One man-equivalent volt is defined as the energy of a condenser of 0.0003 afarad capacitance charged to a potential'of 1 volt (an average man has a capacitance of 0.003 afarady] When the initiator was connected to a conventional blastingcircuit, ignition occurred when a 5 ampere (6.85

volt) current was passed through the bridgewire.

The lack of violence of the initiators was apparent.

Visual inspection of the fired initiators showed that the sides were only slightly bulged whereas the base of the initiator was fragmented. When the initiators were placed base downward on an aluminum plate (1 inch x 1 inch x /2 inch) and initiated, a dent 0.032 inch in diam eter was formed in the plate, indicating the effectiveness of initiation action. However, when the initiators were placed within a hollow tin cylinder two inches in diameter and five inches high, which served to detect missiles, and actuated, the tin cylinder showed no perforations.

The shell may be constructed of a metal, e.g., thin gauge copper, commercial bronze, other copper alloys, aluminum, or of a polymeric material having comparable structural strength and rigidity. As may be ,seen, the thickness of the shell at its closed extremity should be suthcient to retain the charges yet less than that of the side walls of the shell so that upon detonation of the explosive charges, the explosive action will be to a large extent concentrated in the axial direction. The exact length of the shell will depend upon the size of the charges, and the nature of the ignition assembly, and the length of the delay element, if any, to be incorporated in the initiator.

The particular compositions used for the various charges are not critical to the present invention. Accordingly, for the base charge, any of the conventional base charges may be used, e.g., pentaerythritol tetranitrate (PETN), lead azide, cyclotrimethylenetrinitramine (RDX), nitromannite, TNT or cyclotetramethylenetetranitrarnine (HMX). As the priming charge, heat-sensitive detonating compositions such 'as lead azide, diazodinitrophenol, or mercury fulminate may be used. Obviously when squib action is desired, the ignition'c'omposition, the primary charge, and the base charge may be identical.

Advantageously, a variety of commercially available ignition assemblies can be sealed into the shell, for ex ample by means of a resilient plug of the conventional electrical assemblies such as a bridgewire and bead arrangement, a 'bridgewire inserted into a loose 'mass of ignition composition, an exploding bridgewire, and an arc-firing system in which the bridgewire is omitted.

Alternatively, the ignition assembly may be of the nonelectric type wherein a detonation-transmitting cord, such as, for example, low-energy connecting cord as described in US. Patent 2,982,210, or safety'fuse is used toinitiate the priming charge. In this case the cordmay be inserted directly into the shell and the shell sealed about it, as, for example, by crimping. V

Delayed firing action of the initiators, for example for sequential blasting operations, is provided by the delay element of the predetermined interval interposed between :the ignition assembly and the priming charge. These trains comprise a length of defiagrating composition, e.g., boron and red lead, silicon and red lead, or barium peroxide and selenium among'others, in a heavy- Walled tube as oflead or aluminum.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. ..t is therefore to be understood that within the scope "of the appended claims, the invention may be carried but otherwise than specifically'described.

What'is claimed is:

1. An initiator which comprises a tubular shell having two extremities, one of the extremities being integrally closed by a closure member of lesser thickness than the sidewalls of the shell, and an ignition assembly inserted. into and closing the other extremity, the shell'havi'ng positioned sequentially therein from said integral closure member, up to about 1 grain of base'charge, up to about 1 grain of priming charge, and an ignition composition, said "base and priming charges having a cornbiriedcolumnar length of at least about inch and beingin peripheral contact with the shell, the ratio of said length to the diameter of said charges being from about 1.5 to about 3.0.

2. An initiator according to claim 1 wherein a delay element is interposed between said priming charge and said ignition assembly.

3. An initiator as in claim 1 wherein the loading of said priming charge is between about 0.3 and 0.7 grain and the loading of said base charge is between about 0.5 and about 0.7 grain.

4. An initiator 'as in claim 1 wherein the thickness of said closure member is from about 0.003 to about 0.016 inch and the thickness of the said side Walls is from about 0.005 to about 0.030 inch.

5. An initiator comprising a tubular shell having side walls about 0.005 to about 0.03 inch thick integrally closed at one end by a closure member about 0.003 to about 0.016 inch thick, said closure member being thinner than sm'd side Walls, and an ignition assembly inserted into and closing the other end of the shell, said shell containing in sequence from said closure member, between about 0.5 and about 0.7 grain of base charge, between about 0.3 and 0.7 grain of priming charge, and an ignition composition, the base and priming charges having a combined columnar length of at least about A inch and being in peripheral contact with the shell, the ratio of said lengt to the diameter of said charges being from about 1.5 to 3.0, the portion of the shell housing the base and priming charges being of lesser diameter than the portion of said shell housing the ignition assembly.

References Cited in the file of this patent UNITED STATES PATENTS 1,856,431 Rolland May 3, 1932 2,420,201 Seavey May 6, 1947 2,517,763 Cairns Apr. 8, 1950 2,802,421 Horne et a1. Aug. 13, 1957 2,908,559 Zebree Oct. 13, 1959 2,980,019 Noddin Apr. 18, 1961 

1. AN INITIATOR WHICH COMPRISES A TUBULAR SHELL HAVING TWO EXTREMITIES, ONE OF THE EXTREMITES BEING INTEGRALLY CLOSED BY A CLOSURE MEMBER OF LESSER THICKNESS THAN THE SIDEWALLS OF THE SHELL, AND AN IGNITION ASSEMBLY INSERTED INTO AND CLOSING THE OTHER EXTREMITY, THE SHELL HAVING POSITIONED SEQUENTIALLY THEREIN FROM SAID INTEGRAL CLOSURE MEMBER, UP TO ABOUT 1 GRAIN OF BASE CHARGE, UP TO ABOUT 1 GRAIN OF PRIMING CHARGE, AND AN IGNITION COMPOSITION, SAID BASE AND PRIMING CHARGES HAVING A COMBINED COLUMNAR LENGTH OF AT LEAST ABOUT 1/4 INCH AND BEING IN PERIPHERAL CONTACT WITH THE SHELL, THE RATIO OF SAID LENGTH TO THE DIAMETER OF SAID CHARGES BEING FROM ABOUT 1.5 TO ABOUT 3.0. 